This invention relates to an electric circuit for igniting an arcjet thruster and for maintaining an electric arc following ignition. More particularly, a high impedance inductor blocks a high voltage charge from a high voltage source to ignite the arcjet. After ignition, the high impedance inductor magnetically saturates and during steady state operation, a low voltage, high current charge flows through a low impedance inductor.
Arcjets operate by heating a gas with an electric arc and expanding the heated gas through a nozzle to provide thrust. A high voltage, on the order of 5,000-6,000, volts is required to ignite the arcjet thruster and form an electric arc between a cathode and an anode. Once the electric arc is established, the voltage necessary to sustain that arc is much less, on the order of 100 volts. An arcjet thruster thus requires two types of power, a high voltage pulse to ignite the electric arc a and relatively lower constant voltage to maintain the arc.
Several electric starters for arcjets are conventionally used. In one, as disclosed in U.S. Pat. No. 4,766,724 to Gruber, a flyback inductor is in series with the power supply and arcjet thruster. This type of starter must be capable of supporting full load power after startup has occurred. The inductor must be capable of maintaining a fixed minimum inductance during start up and a fixed minimum current during operation. Satisfying both of these requirements with a single inductor will require a large, heavy device. This weight penalty is not desirable in aerospace and outer space applications.
In a second starter, a shunt incorporates a high voltage diode blocking device in series with the power control unit output. The diode must be capable of sustaining full output current from the power control unit after startup. Again, the suitable diode would be heavy and have high internal resistance degrading power control unit efficiency.
In a third alternative, the ignition voltage is provided from a stand alone starter circuit which is removed once the electric arc is established. While such a system is effective for ground based operations such as arc welders, the weight of the separate starter makes this approach impractical for aerospace or outer space applications.
There exists a need for an electric circuit to ignite an arcjet thruster and maintain the electric arc subsequent to ignition which does not have the problems of the prior art.